This invention relates to a tool component and more particularly a tool component for a cutting tool.
The high speed turning of difficult-to-machine materials such as nodular cast irons, steels and iron-, cobalt- or nickel-based superalloys is an important industry. These materials contain iron, nickel or cobalt and have in common that they develop high machining temperatures even at moderate cutting speeds. Thus, thermally activated processes such as adhesion, diffusion or chemical reactions contribute significantly to overall wear of a cutting edge used to turn these materials. Furthermore, the materials have an intrinsic high hardness or are work hardened during machining, or contain hard particles requiring tool materials which have an appreciable hot-hardness characteristic.
PCD (polycrystalline diamond) cannot be used due to the high affinity of carbon with iron, cobalt and nickel based materials which leads to rapid tool degradation once cutting temperatures of 600-700xc2x0 C. are exceeded.
Tungsten carbide is only suitable at low cutting speeds since it has a high reactivity with iron, cobalt and nickel and begins to soften at machining temperatures around 700-800xc2x0 C.
Other ceramic cutting tools have a better hot-hardness than tungsten carbide and can be chemically stable. Alumina-based tools are the most chemically stable materials but they exhibit low thermal conductivity and a relatively poor thermal shock resistance. Si3N4 and Sialon-based tools have a better thermal conductivity and thermal shock resistance but are less chemically stable. Another limiting factor of ceramic cutting tools is their intrinsic brittleness.
PCBN (polycrystalline CBN) is probably the most suitable existing tool material to machine the above difficult-to-machine materials. It is more chemically stable than diamond in combination with the identified workpieces and also has a high abrasion resistance up to high machining temperatures. However, even PCBN tools fail at high speeds through thermally activated chemical wear.
A certain improvement in the wear resistance of carbide and PCBN when machining ferrous difficult-to-machine materials has been achieved by applying thin chemically inert coatings made of materials such as TiN, Al2 O3. HfN, TiAlN and the like. Such coatings act as a diffusion barrier and reduce tool dissolution into the workpiece. However, these coatings principally address crater wear at the tool-chip interface and help little in reducing, flank wear.
According to the present invention, a tool component comprises an abrasive element presenting a cutting edge or point and a thermally conductive element having a thermal conductivity greater than that of the abrasive element in thermal contact with the abrasive element so as to conduct heat generated at the cutting edge or point away from such edge or point.
The thermally conductive element will have a high thermal conductivity, preferably exceeding 120 W/m.xc2x0 K. The use of a thermally conductive element ensures that heat generated at the cutting edge or point is conducted away from that edge or point as rapidly as possible. The thermally conductive element may be in thermal contact with a larger body of another material of high thermal conductivity.
The abrasive element may be PCBN, ceramic, PCD or CVD diamond. Examples of suitable ceramic materials are carbides, nitrides, borides, oxides or silicides of metals of Group IVb, Vb, or VIb. Also suitable as ceramics are silicon carbide, silicon nitride or an oxide, boride or nitride of aluminium. The preferred ceramic materials are alumina, aluminium carbide, titanium carbide, titanium nitride and silicon nitride.
The thermal conductivity to the thermally conductive element will be greater than that of the abrasive element. Thus, in the case of the abrasive element being PCBN, the thermally conductive element may be CVD diamond or PCD. Further, when the abrasive element is PCD, the thermally conductive element may be CVD diamond, single crystal or polycrystalline.
In the tool component of the invention, the thermally conductive element may be a layer in thermal contact with a surface of the abrasive element. In one form of the invention, the abrasive element has a surface leading to a cutting edge or point and the thermally conductive element is bonded to that surface. In another form of the invention, the abrasive element has a surface leading to the cutting edge or point and an opposite surface and the thermally conductive element is bonded to the opposite surface. In these forms of the invention, the abrasive element and the thermally conductive element will generally both have a layer form.
The abrasive element may also form an insert located in a recess in the thermally conductive element.
The thermally conductive element may be uniform in composition, or have regions differing in thermal conductivity. When the regions differ in thermal conductivity, the regions may increase in thermal conductivity away from the abrasive element. The regions differing in thermal conductivity may also be alternating regions. Such regions may be regions of a material having a thermal conductivity greater than that of the abrasive element which alternate with regions of a material having the same or lower thermal conductivity than that of the abrasive element. The alternating regions may be in strip form or in spiral or concentric ring form.
The tool component of the invention may be bonded to a substrate such as a cemented carbide substrate. The cemented carbide substrate will typically be a tungsten carbide substrate. The thermally conductive element will generally be sandwiched between the abrasive element and the substrate.
The invention further provides a method of cutting or turning a material including the steps of providing a cutting edge or point of a tool component as described above, causing relative movement between the cutting point or edge and the material and advancing the cutting edge or point into the material. The advancement of the cutting edge or point into the workpiece can be achieved by moving either the workpiece, the cutting edge or point, or both. This method has particular application to the cutting of materials containing iron, nickel, or cobalt with PCBN or ceramic.